Yellowstone River, named for the ochre sandstone bluffs on the lower Yellowstone, is the longest undammed river in the lower 48. From its source at the slopes of Yonts Peak to its confluence with the mighty Missouri, the Yellowstone stretches 692 miles (1,114 km) and flows through three U.S. States, Wyoming, Montana and North Dakota. William Clark explored the Yellowstone River in 1806. It feeds and drains Yellowstone Lake. Yellowstone River drops 308 feet (94 m) at Lower Yellowstone Falls. The first expeditions into present-day Yellowstone National Park followed the Yellowstone River. And while the Yellowstone is now used for irrigation and recreation, it was once an important transportation route for white settlers and Native Americans.